How to use overlayfs on Linux

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1 What Is OverlayFS

OverlayFS is a union mount filesystem implementation for Linux. It allows a virtual merge of two partitions, while keeping their actual contents separate. One partition is the rootfs partition and the other is the data partition. It has the following advantages:
1) you can easily restore a system's factory settings by formatting the data partition;
2) you can still boot your system since the rootfs is read-only even when the data partition cannot be correctly mounted due to unexpected shutdown.

2 FriendlyELEC's Systems That Support OverlayFS

2.1 Hardware Systems

H3, H5, S5P4418, S5P6818, RK3399 based boards

2.2 OS Systems

FriendlyCore, FriendlyDesktop, Lubuntu

3 How to Check Whether OverlayFS Is Working

Run the df command. If the "/" partition is mounted as "overlay" it means OverlayFS is working;

pi@NanoPi-M1-Plus:~$ df
Filesystem     1K-blocks    Used Available Use% Mounted on
udev              474848       0    474848   0% /dev
tmpfs             102304    3564     98740   4% /run
overlay         28925547 1315493  26112465   5% /
tmpfs             511512       0    511512   0% /dev/shm
tmpfs               5120       4      5116   1% /run/lock
tmpfs             511512       0    511512   0% /sys/fs/cgroup
/dev/mmcblk0p1     40862   11852     29010  30% /boot
tmpfs             102304       0    102304   0% /run/user/1000

4 Partition Settings With OverlayFS

If OverlayFS is enabled in your system there will be three partitions:rootfs, data and boot, and these partitions correspond to a rootfs.img, a userdata.img and a boot.img separately:

root@NanoPi-M1-Plus:~# lsblk /dev/mmcblk0
NAME        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
mmcblk0     179:0    0 29.7G  0 disk
|-mmcblk0p2 179:2    0  1.2G  0 part
|-mmcblk0p3 179:3    0 28.5G  0 part
`-mmcblk0p1 179:1    0   40M  0 part /boot

The system data will be in the rootfs partition and the data written to the root directory will be in the data partition.

5 Restore Factory Settings With OverlayFS

5.1 Method 1: Operate in Userdata Partition

factory reset:

mount /dev/mmcblk0p3 /media/ && cd /media/
mv root/ deleteme.root
mv work/ deleteme.work
reboot

cleanup after reboot:

mount /dev/mmcblk0p3 /media/ && rm -rf /media/deleteme.*

5.2 Method 2: Restore by Setting Commandline Parameters

Add "wipedata=yes" to the kernel's commandline parameters. For S5P4418 and S5P6818 based boards you can do it after system boots;

5.2.1 Restore Factory Settings by Removing Data in Data Partition on System Reboot

sudo fw_setenv bootargs "`cat /proc/cmdline` wipedata=yes"
sudo reboot

5.2.2 Keep Data in Data Partition on System Reboot

sudo fw_setenv bootargs "`cat /proc/cmdline` wipedata=no"
sudo reboot

For other SoC based boards you need to make changes in the kernel commandline by following the methods supported by their BSPs or by entering uboot to proceed.

6 Disable OverlayFS

If you have an installation TF card, use the fdisk command to delete the data partition.